Field of the Invention
The invention relates to a method for avoiding misdetection in a diagnosis of a tank venting system for a motor vehicle driven by an internal combustion engine, which includes delivering a signal corresponding to the pressure in the tank venting system from a pressure sensor to a control unit, and assessing the tank venting system as to its operability on the basis of a pressure course over time.
The goal of such tank venting systems is to prevent hydrocarbons from evaporating out of the fuel tank into the atmosphere.
To that end, the tank venting system generally has a fuel tank and a tank venting valve, which communicates with an intake manifold of the internal combustion engine that drives the motor vehicle. As a result, with the aid of negative pressure in the intake tube, fuel vapors can be aspirated off and delivered for combustion in the engine cylinders. Typically, the volume in the fuel tank located above the fuel is not aspirated off directly. Instead, the fuel vapor as a rule is temporarily stored in a separate container that contains an adsorbing material, which as a rule is an activated charcoal filter. That prevents the fuel vapor from escaping into the environment. The activated charcoal filter adsorbs fuel vapors in periods of time in which no aspiration from the intake tube occurs, for instance when the engine is stopped, or whenever the tank venting valve is kept closed on the basis of the current operating state of the engine.
Since the activated charcoal filter can only store a limited fuel mass, it must be flushed in suitable engine operating ranges. In that process the tank venting valve, which is disposed in a line between the activated charcoal filter and the intake tube of the engine, is opened, by triggering with suitable signals from an electronic control unit of the engine. The opening cross section of the tank venting valve and therefore the flushing flow through the activated charcoal filter can be adjusted by varying the triggering duty cycle of that signal.
There is a risk that such a tank venting system will leak, or that components in the system will not function properly. Such systems are therefore repeatedly checked for operability during motor vehicle operation. One method for checking a tank venting system is described, for instance, in German Published, Non-Prosecuted Patent Application DE 44 27 688 A1, corresponding to U.S. application Ser. No. 08/510,744, filed Aug. 3, 1995. Through the use of that method, the tank venting system is evacuated by the negative pressure prevailing in the engine intake tube, and the system is assessed on the basis of negative pressure buildup and negative pressure reduction gradients.
The course and outcome of the diagnosis of the tank venting system through the use of vacuum processes, whether with the aid of the intake tube negative pressure or through the use of an external vacuum pump, are greatly influenced by the fill level of the fuel tank. When the fuel tank is full, as a rule leaks with smaller cross sections are more easily detected than, for instance, in a tank that is only half full, so that the fill level represents a test condition for the FTP test. The result is undesired diagnostic entries (only leaks with cross sections larger than 1 mm need to be detected) and subsequent demands for assurance that are not actually necessary.
In special variants of tank venting systems, two-way valves are installed between the fuel tank and the activated charcoal filter in order, among other goals, to generate a pressure buildup when the tank is full and thus to force the gas pump nozzle to shut off early enough when the motor vehicle tank is being filled. At that moment, a float valve closes the second connecting hose, which has no valve, between the fuel tank and the activated charcoal filter in order not to cause pressure losses, in what are known as on-board refueling vapor recovery systems (ORVR systems), in which the fuel vapor arising in refueling is carried from the fuel tank into the activated charcoal container through a line. Problems then arise in the diagnosis of the tank venting system when the fuel tank is full to the top, because the two-way valve becomes operative then (pressure loss in generating negative pressure), and the result can be a misdiagnosis. That problem can be solved, for instance, by bypassing the two-way valve with an electrically triggerable bypass valve during the diagnosis of the tank venting system.